Prior Art Statement
The following publications are representative of the most relevant prior art known to the Applicant at the time of filing the application.
______________________________________ UNITED STATES PATENTS 3,356,513 December 5, 1967 M. E. Washburn 3,639,101 February 1, 1972 M. E. Washburn 3,679,444 July 25, 1972 M. E. Washburn 3,892,840 July 1, 1975 Jorgen Abildtrup 4,069,058 January 17, 1978 M. E. Washburn FOREIGN PATENTS 2,221,421 October 11, 1974 France ______________________________________
The Washburn U.S. Pat. No. 3,356,513 discloses processes for synthesizing silicon oxynitride and shaped articles of that material. Powdered silicon oxynitride of relatively high purity is prepared by reacting from 47.5 to 98 wt.% of silicon powder with 1 to 50 wt.% of fine silica in the presence of up to 5 wt.% of an alkaline earth or rare earth oxide. The mixture is fired to about 1350.degree. C. in an atmosphere of N.sub.2 and O.sub.2 wherein the ratio of O.sub.2 to N.sub.2 is from 1 to 99 to 6 to 94 parts by volume. The particle size of the starting powders is preferably 20 microns and finer. To form silicon oxynitride shapes by the so-called reaction bonding method, the above mixture of powders is cold pressed to the desired shape and fired in an air-N.sub.2 atmosphere at 1450.degree. C., the particles size of the powders being preferably 20 microns and finer. The resulting article has a desnity of 1.97 g/cm.sup.3. In another embodiment, silicon oxynitride powder synthesized as described, is crushed so as to pass through a 100 mesh screen (U.S. Standard Sieve Series). The powder is hot pressed at 1700.degree. C. under 6000 p.s.i. for 13 minutes. The resulting piece has a density of 2.7 g/cm.sup.3 i.e. about 96% of the theoretical density.
U.S. Pat. No. 3,639,101 is relevant for its teaching that all of the O.sub.2 needed for the silicon oxynitride forming reaction can be added as a solid to the mix, in the form of silica. The highest density product which is produced by reaction bonding is 2.01 g/cm.sup.3.
A method of forming reaction bonded silicon oxynitride products is taught by U.S. Pat. No. 3,679,444 wherein growth and shrinkage is controlled by using varying amounts of very fine silica in the Si-SiO.sub.2 -reaction promoter raw batch mix. A range of 2 to 15% by weight of the raw batch mix advantageously is silica fume (particle size of 0.5 microns or less), the normal total amount of silica being about 40% by weight. It is also taught that where high density products are desired and dimensional stability is not critical, all of the silica in the mix may be present as silica fume.
The Abildtrup reference U.S. Pat. No. 3,892,840 discloses a method of forming silicon oxynitride products free of silica. This is accomplished by heating a preformed silicon shape, wherein the silicon particle size is in accordance with FEPA Standard F320/29 or finer, between 1200.degree. and 1600.degree. C. in an O.sub.2 -N.sub.a atmosphere with a partial pressure of O.sub.2 lower than 10.sup.-17 atmosphere.
The Washburn patent U.S. Pat. No. 4,069,058 is concerned with bonding silicon oxynitride grain that is relative coarse, with fine crystals of silicon oxynitride. It is relevant for its disclosure of the use of fine (2 microns) silicon and the use of reaction or sintering aids such as calcium and magnesium fluorides.
French Pat. No. 2,221,421 to Mary et al involves a method for the manufacture of silicon oxynitride articles with densities as high as 92% of theoretical. This is accomplished by mixing free silicon oxynitride powder with an additive or reaction aid such as an alkaline earth oxide or an oxide of yttrium or aluminum and cold pressing the powder to the desired shape. The green shape is then coated with silicon oxynitride powder and fired at 1600.degree. C. The shell or coating of silicon oxynitride is then chipped off or otherwise removed.